Fluid power systems utilizing a petroleum oil as the working fluid are widely used on stationary and mobile industrial machinery. A basic fluid power system is comprised of a positive displacement fluid pump, a working piston in a cylinder, a fluid reservoir, interconnecting piping and a working fluid. Working fluid from the reservoir is pumped under pressure to the cylinder. The fluid pressure against a piston in the cylinder causes movement of the piston, thus converting some of the fluid pressure to work, and the spent fluid is returned to the reservoir. The reservoir is normally vented to ambient atmospheric pressure and the spent fluid, flowing under some pressure above atmospheric pressure, spews into the reservoir causing turbulence, and sometimes foaming, of the fluid in the reservoir. Reservoir air, displaced by the returning fluid, is vented to ambient atmosphere through a short standpipe capped by a steel wool packed breather cap similar to those used for venting automotive engine crankcases.
The turbulence in the reservoir fluid often results in working fluid being carried up the standpipe with the expelled air and out of the reservoir through the breather cap. Such breather caps are designed to inhibit the flow of dust and dirt contaminants into the reservoir but not to prevent the flow of fluid out through the cap. The fluid escaping the reservoir in this manner is wasted and causes equipment cleanliness problems.
On mobile construction machinery, transportation equipment and, particularly, fork lift trucks, the resulting oily mess is particularly undesirable. Machinery cleanliness is often essential, as where a fork lift truck is used in a clean warehouse or manufacturing facility. The mobile nature of such machines adds to the turbulence in the reservoir and increases the fluid spillage problem.